1. Field of the Invention
The present invention relates to a spindle motor rotating a recording disk and a recording disk driving device comprising the spindle motor.
2. Background Art
In a hard disk drive (HDD) which is one of large-capacity information recording devices, the storage capacity per unit area of a disk plate as a recording medium thereof has been increased. Therefore, the information storing capability of HDD has been increased, and the size has been reduced.
Corresponding to this, with regard to applications of HDD, in addition to a conventional application as an auxiliary memory of a computer, opportunities in which a hard disk is used for a video/audio equipment or a portable equipment have been increased. Furthermore, a computer itself has more often been used in a quiet environment such as in office and at home. Therefore a less noisy HDD as compared to the conventional one has been sought. In order to meet these demands, for a bearing of the spindle motor (hereinafter, merely refer to a motor) mounted on the HDD, a fluid dynamic bearing (FDB) has been used instead of a ball bearing (BB) having large sliding noise. In addition, in order to achieve miniaturization and reduction in thickness of the HDD and realize a low cost, a base plate in which a part of a case of the HDD and a bracket composing a base part of the motor are integrally formed has been used.
In the HDD using the FDB and the base plate, noise has been remarkably decreased as compared with the conventional BB, while high frequency noise, which was inconspicuous due to the sliding sound of BB, all the more has posed a problem in relation to a natural frequency of the base plate.
Even at the same sound pressure level, a loudness of a sound that can be captured by human auditory sense differs depending on the frequency of the sound. In particular, it is known that sound of 1000 to 7000 Hz tends to sound larger as compared with that in another range, which is noise discomforting a human. It is known that the frequencies that human auditory sense can recognize are about 20 to 20000 Hz. In the HDD using the FDB and the base plate, it is a problem to reduce, in particular, a noise frequency range of 1000 to 7000 Hz.
Causes of Noise—As a major cause of generating such noise having the frequency range of 1000 to 7000 Hz, there is a switching noise of a brushless motor. The spindle motor mounted on the HDD is a DC brushless motor, in which by switching a current through a coil in forward and backward directions in a constant period depending on a rotational speed of the motor, a rotating magnetic field induced by the switching current according to the switching period is generated in a field around a stator which the coil is wound around. The rotating magnetic field generates a magnetic attracting force and a magnetic repulsive force together with a magnetic field formed by a rotor magnet to thereby rotate a rotor.
All the magnetic attracting force and the magnetic repulsive force are not used to rotate the rotor but a part of these forces are used to generate a periodic vibration. Such vibration is referred to as an electromagnetic vibration. This vibration includes a plurality of different frequency components, and to obtain which frequency has large vibration energy, a waveform of the vibration is extended into Fourier series and we can find each frequency coefficient of Fourier series which corresponds to vibration energy of the frequency.
For the spindle motor, a three-phase brushless motor is generally used and the electromagnetic vibration generated in the rotor magnet has three peaks at the frequency order obtained by multiplying the number of poles of the rotor magnet by three, that is, the number of the phases, at the frequency orders of its integer multiples (these are referred to as “basic orders”) and at the frequency orders before and after the above-mentioned orders. For example, in the case of a four-pole rotor magnet, they are 11th, 12th, 13th, 23rd, 24th, 25th, 35th, 36th, 37th, 47th, 48th, 49the like. Here, the order denotes an integer value obtained by dividing each frequency component of the vibration frequency by the number of revolution. It is known that the number of poles of a general rotor magnet is 4 to 24 and the vibration has a plurality of amplitude peaks in a range of about 11th to 350th frequency orders, in particular, in a range of 20th to 100th. This frequency of vibrations is about 1200 to 20000 Hz in a motor performing steady rotation of 3600 to 12000 per minute, which corresponds to sound from a range that can be heard well by human auditory sense to the highest range that can be recognized by human auditory sense. That is, at least one of the causes of generating uncomfortable noise generated from the base plate is this electromagnetic vibration. This electromagnetic vibration is generated in the rotor magnet and the stator, respectively.
As causes of generating, in particular, the noise of 1000 to 7000 Hz in the HDD using the above-mentioned FDB and base plate, the following two are considered.
First cause of noise generation—Firstly, among this electromagnetic vibration generated in the rotor magnet, there is a vibration transmitted to the base in non-contact. This vibration corresponds to the electromagnetic vibration having peaks at the frequency orders before and after the basic orders, for example, in the case of the four-pole rotor magnet, the 11th, 13th, 23rd, 25th, 35th, 37th, 47th, 49th orders and the like.
The rotor magnet is one of sections where the electromagnetic vibration is generated, and the vibration generated in the rotor magnet is transmitted to the FDB unit through a rotary member. In the FDB unit, a lubricating fluid is interposed between the rotary side and the stationary side in members composing the FDB unit and the FDB unit is supported in a non-contact state. The member on the rotary side and the member on the stationary side composing the FDB unit are opposed to each other through a very minute clearance, and further a dynamic pressure of the lubricating fluid increased by rotation generates sufficient rigidity to support a shaft. Therefore, even if the FDB unit supports the shaft in non-contact by interposing the fluid, the vibration which has been generated in the rotor magnet and transmitted to a bearing through the rotary member is partially transmitted to the base plate.
Conventionally, the base plate has been directly attached to the FDB unit. Therefore, the vibration generated in the rotor magnet is transmitted to the FDB unit and also to the base plate. When the frequency of the vibrations coincides with both a natural frequency and a vibration mode that the base plate has, a resonance occurs to generate a large noise.
Second cause of noise generation—A second noise is generated by another electromagnetic vibration which is caused by the same as in the first cause and transmitted to the base in contact. Among the electromagnetic vibration, there is a vibration directly transmitted from the stator to the base plate. The electromagnetic vibration having peaks at the above-mentioned basic frequency orders corresponds to this, for example in the case of a four-pole rotor magnet, the electromagnetic vibration of 12th, 24th, 36th, 48th orders and the like correspond to this.
This vibration is also similar to the vibration generated in the rotor magnet, and is about 1200 to 20000 Hz, which corresponds to the sound from the range that can be heard well by human auditory sense to the highest range that can be recognized by the human auditory sense. The base plate is directly vibrated by this stator, and when the natural frequency of the base plate is close to the number of vibration of the stator, the resonance occurs, thereby generating a large noise.
Problems—In order to eliminate these causes of vibration, one way is to decrease the natural frequency of the base plate or to prevent the frequencies of the electromagnetic vibration generated by the stator and the rotor magnet from coinciding with each other, which allows resonance not to occur.
However, the reduction in thickness and the miniaturization of HDD make it difficult to freely change the design of the base plate, and the natural frequency becomes higher and higher. Furthermore, since the number of poles of the rotor magnet, the number of magnetic poles of the stator and the like largely affect other properties such as the torque of the motor, it has been difficult to easily change the design.
On the other hand, as the applications of HDD are increased, the model and specification of a motor used for HDD become more and more various. It is difficult with one or a few basic models to satisfy respective conditions such as the size, a number of discs, a number of revolutions, and use environments where recording disks are used. Therefore, when a member composing a motor including the FDB unit is changed according to changing the specification, the whole motor including the base plate, a facility for producing the motor such as jigs and the like may need to be remade. In particular, the base plate is manufactured by using a die casting or press working, and thus a very high-cost metal mold should be changed to change a shape of the base plate. Furthermore, high processing accuracy and durability are required of the jigs, which are very expensive. In addition, an increase in the number of types makes not only the maintenance difficult but also the handling complicated.